1. Field of the Invention
The present invention relates to a controlling apparatus and method of an inverter to drive a three-phase motor.
2. Description of the Related Art
A three-phase motor with an inverter is commonly used for home appliances, such as an air conditioner, a washer, a refrigerator, and so on, considering energy efficiency. Such inverter mostly uses a PWM method (Pulse Width Modulation), and, especially, a SVM (or SVPWM, Space Voltage Vector Pulse Width Modulation) method. The SVM method is widely known for an ability to maximize an amount of usable DC (Direct Current) voltage connected to the inverter. In addition, the SVM method remarkably reduces current harmonic components at a normal state as compared with a triangular PWM which is widely used in general. See Analysis and Realization of a Pulse Width Modulation on Voltage Space Vector, written by H. W Van der Broeck and H. C Skydelny, IEEE Trans. On Appl., 1998, vol IA-24, no. 1, p. 142-150; and Analysis of the Harmonics in Voltage fed Inverter Devices Caused by PWM Schemes with Discontinuous Switching Operation, written by H. W Van der Broeck, 1991, EPE Conf Rec-3, p.261-266.
However, the conventional SVM has shortcomings of a time-consuming calculation and a complicated realization. An actual switching time is determined based on eight switching conditions of the inverter likely generating under the conventional SVM method. According to such method, two valid switching vectors in a vector space are set nearest to a standard voltage vector, and then each vector is applied within a certain time calculated by an average of each cycle to create an actual switching pattern, and the certain time is used to calculate the actual switching time. That is, after specifying a sector from the location of the standard vector, a vector is selected to be applied and a time for applying is calculated, and then the calculated times are added together.
FIG. 1 through FIG. 3 illustrate a method of inverter control based on the conventional SVM method.
FIG. 1 illustrates an inverter controller having a conventional current controller using a dq-rotating coordinate system.
As shown in FIG. 1, the conventional inverter controller includes an inverter 101, a dq converter 102, a current controller 103, a SVM algorithm module 104, and a gate driver 105.
The inverter 101 has three first switches (Sup, Svp, and Swp) connected between each of phase terminals (u, v, and w) and a positive terminal of a DC (Direct Current) power (Vdc), and three second switches (Sun, Svn, and Swn) connected between each of the phase terminals (u, v, and w), and a negative terminal of the DC power.
FIG. 2 illustrates an allocation of conventional voltage vectors in a space coordinate. The inverter 101 can have eight voltage vectors by means of defining a state of each switch as 1 or 0. The state 1 is set when each of the first switches (Sup, Svp, and Swp) in each of the phases is turned on having the second switches (Sun, Svn, and Swn) turned off. The state 0 is set when each of the first switches in each of the phases is turned off having each of the second switches turned on.
A vector v(t) describes line voltages in the motor as vd and vq, respectively, centering on the d-axis and q-axis of the dq-rotating coordinate system and synchronizing a size (V) of an adjacent voltage vector as a phase angle (θ) to an adjacent voltage vector. Each vector region is a phase-shift space from each of the voltage vectors to the adjacent voltage vector, respectively.
FIG. 3 illustrates a relationship between the vector regions and the phase voltages. A region I starts from where a phase voltage Vuo has a highest value and a region III starts from where a phase voltage Vvo has the highest value.
The d-p converter 102 receives the line voltage flowing from each of the phase terminals to the motor, and sends out the line voltage to the current controller 103 after shifting a coordinate of the received line voltage using the rotating coordinate system. The current controller 103 calculates components of an output voltage in the rotating coordinate system. The SVM algorithm module 104 calculates an output voltage vector and determines a vector holding time of the adjacent vector after setting the adjacent voltage vectors and the corresponding vector regions, and sends out the output voltage vector to the gate driver 105. The gate driver 105 receives the vector holding time and sends out gate signals of six switches in each phase of the inverter 101.
Hereinafter, a calculation of a switch continuity time, namely, a time for a switch-on state to send out a vector v (t) will be described referring to FIG. 2.
To send out the vector v (t), the switch continuity time has to be modulated responding to the size (V) of the vector and the phase angle (θ).                     V        =                                            Vd              2                        +                          Vq              2                                                          (        1        )                                θ        =                              ω            ⁢                                                   ⁢            t                    +                      θ            ⁢                                                   ⁢            d            ⁢                                                   ⁢            q                                              (        2        )                                (                  Herein          ,                                    θ              ⁢                                                           ⁢              d              ⁢                                                           ⁢              q                        =                                          tan                                  -                  1                                            ⁢                                                V                  ⁢                                                                           ⁢                  q                                                  V                  ⁢                                                                           ⁢                  d                                                                    )                                         
The region to specify the vector adjacent to the output vector for the modulation is calculated as follows.                                                                                           θ                  ⁢                                                                           ⁢                  v                                =                                  θ                  -                                                            II                      3                                        ⁢                    n                                                              ,                                                                                                                                                       for                  ⁢                                                                           ⁢                  n                                =                1                            ,              2              ,              …              ⁢                                                           ,              6                                                          (        3        )            
To control currents, the voltage vector can be modulated as follows.                     v        =                                                            T                ⁢                                                                   ⁢                α                                            T                ⁢                                                                   ⁢                s                                      ⁢            V            ⁢                                                   ⁢            α                    +                                                    T                ⁢                                                                   ⁢                β                                            T                ⁢                                                                   ⁢                s                                      ⁢            V            ⁢                                                   ⁢            β                                              (        4        )            
With following trigonometric equations, continuity times Tα for the adjacent voltage vectors Vα, Tβ for Vβ, and To for the origin vector Vo can be obtained.                               T          ⁢                                           ⁢          α                =                                            V              ⁢                                                           ⁢                              3                                                    V              ⁢                                                           ⁢              α                                ⁢          T          ⁢                                           ⁢          α          ⁢                                           ⁢                      sin            ⁡                          (                                                II                  3                                -                                  θ                  v                                            )                                                          (        5        )                                          T          ⁢                                           ⁢          β                =                                            V              ⁢                              3                                                    α                                ⁢          T          ⁢                                           ⁢          β          ⁢                                           ⁢          sin          ⁢                                           ⁢                      θ            v                                              (        6        )                                To        =                  Ts          -                      T            ⁢                                                   ⁢            α                    -                      T            ⁢                                                   ⁢            β                                              (        7        )            
However, such conventional technology has complicated algorithms which specifies the vector regions, and calculates the switch continuity times of the adjacent voltage vectors, and sets a pattern per region to calculate the switch continuity times. Also, the gate driver continuously switches the six switches of each phase in the inverter 101.